This invention generally relates to photosensitive articles that include a stabilized diazonium composition and a method associated therewith. More particularly, the invention relates to the use of purine derivatives in preparing plates or photoresists, which purine derivatives are incorporated into a diazonium composition that is utilized in making the plate or photoresist. Preferably, this diazonium composition also includes a supplementary stabilizer component, such as a zinc salt including inorganic salts such as zinc chloride, or preferably a low molecular weight alkyl sulfonic acid zinc salt. A polymer binder may also be included within the diazonium composition in order to assist in maintaining the composition as an emulsion.
It has long been known to utilize diazonium materials in connection with the preparation of products such as lithographic printing plates and photoresists such as are provided relative to printed circuit boards. For convenience, when reference is made hereinafter to a "plate", that term is intended to include products having generally rigid substrates and diazonium compositions coated thereonto, including lithographic printing plates, photoresists and the like. Often, such plates must be stored for extended periods of time between preparation of the plate and its use when same is selectively exposed and developed or otherwise put into use. When subjected to storage for extended periods of time, there is a tendency for the diazonium material of such plates to undergo reactions that detract from the overall sensitivity of the diazonium material present on the plate.
Previously practiced approaches for stabilizing these plates include reacting the diazonium material with a complexing agent in order to "tie up" the reactive diazonium group while attempting to retain a substantial amount of the light sensitivity of the diazonium material. Typically, these stabilization techniques are intended to enhance the resistance of the diazonium material to environmental moisture which can lead to undesirable physical and chemical changes of the diazonium material during extended periods of storage. Other approaches include interposing a stabilizer layer between the diazonium material and a substrate component or coating which would adversely affect the diazonium compound.
Diazonium material stabilization is believed to inhibit, under storage conditions, reactions by which the diazonium groups thereof are degraded and rendered less sensitive to light. In this case, such a reaction results in a premature "tanning" of the diazonium material over the general surface area of the plate, thereby reducing the contrast between the background area and the areas of the plate that are selectively exposed at the time that the plate is put into use by exposure and development in order to form the desired image on the surface of the plate. Stabilization of diazonium materials which are coated upon plates that are to be stored for extensive time periods should take into consideration the reactivity of the diazonium group itself. Plate stabilization should also consider the tendency of many diazo condensates to undergo dimerization, trimerization and/or cross-linking, which forms polymers that, because of their increased molecular weight, are insoluble even though they include reactive diazonium groups. Because of this, the solubility differential between the exposed and unexposed areas is substantially reduced when the plate is subsequently used, which phenomena is commonly known as "scumming."
Plates that are stabilized in this manner are prepared in accordance with the present invention by providing a diazonium sensitizer composition that includes a diazonium compound and a stabilizing system that includes a purine derivative, especially a theophylline derivative, preferably in combination with a supplementary stabilizer component such as a salt of a low molecular weight alkyl sulfonic acid or an inorganic zinc salt. An emulsion polymer binder may also be included to assist in stabilizing the sensitizing solution, typically while enhancing the ink receptivity of the diazonium composition.
It is accordingly a general object of the present invention to provide a plate of improved stability.
Another object of this invention is an improved lithographic printing plate and method of providing same in an unusually stable condition.
Another object of this invention is to provide a photoresist having enhanced stability.
Another object of the present invention is to utilize a purine derivative, especially a theophylline derivative, as a plate constituent for enhancing the shelf stability thereof.
Another object of the present invention is to provide an improved additive or subtractive negative working plate which has an especially stable diazonium material that is rendered insoluble upon exposure to actinic radiation.
Another object of this invention is to provide an improved additive or subtractive positive working plate that includes a stabilized diazonium material that is rendered soluble upon exposure to actinic radiation.
These and other objects of the present invention will be apparent from the following further deatiled description of this invention.
Plates, especially those intended for use in lithographic or planographic printing processes, include a plate base or support member, a layer and/or treatment for rendering the plate base or support member more suitable for use as a plate, and one or more layers of a sensitizer coating that is photopolymerizable in response to actinic radiation. When desired, protective coatings can also be included. The photopolymerizable sensitizer layer or layers of the plate in accordance with this invention include a diazonium material that is laid down from a diazonium composition which includes a stabilizing system. Usually, the stabilizing system will be included in the diazonium composition in order to inhibit what are believed to be chemical reactions including dimerization and/or trimerization of diazo condensates and the regressive reaction of diazonium groups thereof into precursors of the diazonium group such as --NH.sub.2, --NO.sub.2 and/or --NO groups.
The stabilized diazonium composition according to this invention includes the desired diazo resin material that is responsive to actinic radiation, a purine derivative, especially a theophylline derivative, preferably in combination with a supplementary stabilizer component such as a salt of a low molecular weight alkyl sulfonic acid or an inorganic salt stabilizer.
Typically, the support member or plate base is a flexible sheet-like material composed of metal, plastic, or the like. In this regard, it is important to note that the present invention finds advantageous utility with anodized aluminum plate bases. In accordance with known techniques, the anodized plate base is brush grained or otherwise suitably treated to improve its ability to bond with a sub-base coating thereover and then washed to remove oils and contaminants which may be present on the surface thereof after such operations.
Numerous sub-base coating materials are available, including phytic acid, melamine-formaldehyde condensation resin overcoated with polyacrylamide, and thereafter treated with zirconium acetate, urea-formaldehyde, titanium ortho esters, silane-acrylics and a silicate sub-bases such as described in U.S. Pat. No. 2,714,066. In addition, other sub-bases which are typically used on plates include gelatin, polyacrylic acid and water soluble salts thereof, polymethacrylic acid and water soluble salts thereof, carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose, some titanates, modified resins of urea-formaldehyde and melamine-formaldehydes, polyvinyl alcohols, ferrocyanides, bicromates of sodium, potassium and ammonium, and oxides of the metal forming the plate as well as other sub-base materials. Combinations of the above are also included.
Stabilized diazonium compositions in accordance with this invention are polymeric water emulsions that include a stabilizer system and a diazonium material, typically added as an aldehyde condensate. The diazo condensate will be included within the composition at a concentration of between about 0.03 to 3 weight percent, based upon the total stabilized sensitizer composition, a more preferred range being between about 0.1 and about 1 weight percent. The purine derivative of the stabilizer system is included at a concentration of between about 1 and 30 weight percent, based upon the total weight percent of the total composition, preferably between about 2 and 20 weight percent as a total amount of one or more purine derivatimves. When the supplementary stabilizer component is a salt of a low molecular weight sulfonic acid, it is included at a weight percent as high as about 25 weight percent, typically between about 1 and 20 weight percent, based upon the total weight of the composition. When an inorganic salt supplementary stabilizer component is included, it can be included up to its solubility limit. An inorganic salt such as zinc chloride can be included within the composition at a weight percent from 0 to about 1 percent, based upon the total weight of the composition. When a polymer binder is included, such may be included within the emulsion at a weight percent as high as about 25 weight percent, preferably between about 5 and 20 weight percent, based upon the total weight of the composition. The balance of the emulsion composition is a solvent such as water, typically greater than about 45 weight percent and usually no greater than about 90 weight percent or above.
Especially suitable purine derivatives of the stablizing system in accordance with this invention are the theophylline derivatives, particularly those having a structure as follows: ##STR1## wherein R is a hydrocarbon chain of from 1 to 5 carbon atoms and having one or more hydroxy substituents. Typically, R is an alcohol, diol or triol of from 1 to 4 carbon atoms. These compounds are particularly stable within a water emulsion, and such compounds may be used alone or in combination with another such compound within the stabilizing system according to the present invention. It is important that these purine derivatives are readily water soluble or water dispersible. Exemplary theophylline derivatives suitable for use in accordance with this invention are proxyphylline, which is 7-(2-hydroxypropyl)theophylline; dyphylline, which is 7-(2,3-dihydroxypropyl)theophylline; and ethoxyphylline, which is 7-(2-hydroxyethyl)theophylline.
The supplementary stabilizing components are preferably low molecular weight sulfonic acid salts or inorganic salt stabilizers. The low molecular weight sulfonic acid salts are especially preferred because they are particularly advantageous in minimizing the likelihood that the plate will repel a lacquer or developer that is subsequently applied thereto. Other, more traditional, stabilizers such as metal salts like zinc chloride tend to be sparingly soluble within the stabilized diazonium composition according to this invention, which requires a rather strict control regarding the inclusion of such inorganic salts within the stabilized diazonium composition.
The low molecular weight alkyl sulfonic acid salts according to this invention are particularly advantageous because of their effectiveness as a supplementary stabilizer component and because they avoid solubility problems within these stabilized diazonium compositions, which problems are typically encountered with generally related salts such as the aryl sulfonic acids and salts thereof. The alkyl groups of these alkyl sulfonic acid metal salts may be unsubstituted or substituted, for example with hydroxy groups or halogens, and have a carbon chain length of between about 1 and about 6. The alkyl sulfonic acid salts that are the preferred supplementary stabilizer components include metal salts of methane sulfonic acid, of hydroxy ethane sulfonic acid, of monofluoro methane sulfonic acid, of difluoro methane sulfonic acid, of trifluoro methane sulfonic acid, of chlorine-substituted alkyl sulfonic acids, and the like.
It is typically most advantageous to prepare these low molecular weight alkyl sulfonic acid salts in situ within the stabilized diazonium composition. For example, zinc methane sulfonate is advantageously provided to the composition by separately adding, in generally stoichiometric quantities, methane sulfonic acid and a metal oxide, hydroxide or carbonate, such as zinc oxide, zinc hydroxide or zinc carbonate.
Polymer binder components may be included to assist in holding the stabilized diazonium composition together in order to enhance the homogeneity of the coated stabilized diazonium composition. The polymer binder also helps in laying down an especially smooth coating and also provides a protective film and a base to increase the oleophilic properties of the coated stabilized diazonium composition.
In order to simplify and enhance the ability of the polymer binder to be included within the stabilized diazonium composition, it is preferably provided in emulsion form. Generally speaking, these polymer binders may be acetates, acrylates, methacrylates and the like. Examples include acetate emulsions such as ethylene vinyl acetate emulsions, vinyl acetate emulsions and ethylene vinyl acetate copolymer emulsions. Other polymer binders include acrylic resin emulsions and methacrylic resin emulsions including ethyl methyl methacrylate and the like.
Many such polymer binder emulsions include coalescing solvents in order to maintain the stability of the polymer binder emulsion. When desired, coalescing solvent may be added as such to the stabilized diazonium compositions in accordance with this invention. Many coalescing solvents are well-known for assisting in binding emulsions together. Exemplary coalescing solvents include Cellosolve materials and the like, such as 2-methoxyethanol.
The diazo condensates that are included within the stabilized diazonium composition are generally known. They are light sensitive and water soluble or solvent soluble such that they are soluble within the stabilized diazonium compositions according to this invention.
Illustrative examples of diazo resins which are suitable as water dispersible diazo photosensitizers are the condensation products of an aldehyde with a diazo compound having an amine substituent such as, for example, a water soluble condensation product of a para-diazo diphenylamine and formaldehyde, such as the diazo resins manufactured by Fairmont Chemical Co. Typically, such materials are furnished as salts, such as zinc chloride salts.
Other diazo compounds which can be used include the diazo oxides such as pyrido[1,2-a]benzimidazol-8-yl13(4H)-diazo-4(3H)-oxo-1-naphthalene sulfonate. Diazonium compounds which can also be used include such materials as 4'-bromodiphenylamine-4-diazonium chloride; 4'-chlorodiphenylamine-4-diazonium chloride; 4-methoxydiphenylamine-4-diazonium chloride; 3'-methoxydiphenylamine-4-diazonium chloride; 3'-propoxydiphenylamine-4-diazonium chloride; X,X.sub.1,X.sub.2 -tribromodiphenylamine-4-diazonium chloride; 4'-methyldiphenylamine-4-diazonium chloride; 4'-hydroxyldiphenylamine-4-diazonium chloride; 4'-benzoylaminodiphenylamine-4-diazonium chloride; 4'-fluorodiphenylamine-4-diazonium chloride; 2-anilidosulfonyldiphenylamine-4-diazonium chloride; 2-(p-toluene-sulfonyl-amino)diphenylamine-4-diazonium chloride; p-diazo-N-ethyl-N-benzyl aniline 1/2 ZnCl.sub.2 ; 4'-diazo-2-chloro-N,N'-diethylaniline-1/2 ZnCl.sub.2 ; and other similar and well known diazonium materials.
Regarding the method in accordance with this invention, the stabilized diazonium compositions described herein are prepared and coated onto a suitable base plate, typically one that has been treated, such as with a sub-base coating. The diazonium composition is then coated thereover. The stabilizing aspect of this invention is carried out by including the stabilizer system in accordance with this invention within the composition from which the diazonium compound is coated onto the base plate. When a particularly fast plate is desired, the diazonium material should be coated in as thin a layer as possible, even as thin as a generally monomolecular layer or an approximation thereof.